Clutch systems are used in motor vehicles for separating the engine and gearbox. In modern vehicles, virtually exclusive use is made of friction clutches or hydrodynamic transmissions. In the customary friction clutch, the force-transmitting connection between the shaft from the engine and the shaft entering the gearbox is established by means of one or more disks pressed against one another, with at least one clutch disk connected to the shaft entering the gearbox, which has linings on both sides, being located between the flywheel of the engine and a clutch pressure plate. When the clutch pedal is depressed, the pressure plate is pushed away from the clutch disk against the spring force of the disk spring, as a result of which the clutch disk is released. The connection between the engine and the gearbox is thereby interrupted.
The magnitude of the torque which can be transmitted at a given static friction depends on the area and the diameter of the clutch disk and on the spring force of the disk spring. For large torques, clutch systems having a relatively large friction area or multidisk clutches are therefore used. The clutch disk is, especially on starting, subject to wear by rubbing and is heated by the friction. This heating can reduce the static friction and thus the torque which can be transmitted. Local heating also leads to nonuniform contact of the surfaces and thus to grabbing.
In view of this prior art, it is an object of the invention to provide clutch systems which display very little dependence of the static friction on the operating temperature and whose force locking via static friction is also independent of the twisting angle.
Friction pairings with ceramic materials, in particular fiber-reinforced ceramic materials, are known. When used as brake disks and brake linings, high thermal stability, high energy consumption and a coefficient of friction for sliding friction which has a low dependence on the load are required. In the case of clutch systems, high static friction and high torsional stiffness and also force locking due to static friction which is independent of the twisting angle are wanted.
During slipping of the clutch, force transmission occurs by sliding friction between the surfaces of the friction pairings. On changing from sliding friction to static friction, a transition which is not very abrupt which both contributes to the comfort of the clutch and avoids sudden torsional stress on the gearbox shaft is desirable. In the case of the materials used hitherto, long-term operation in a slipping state of the clutch slipping was not possible since, firstly, the strong heating in the case of conventional materials led to a change in the coefficient of friction of the friction pairing (these fluctuations in the coefficient of friction of the friction pairing lead to the well-known “grabbing”) and, secondly, the demands made on the thermal stability of the surrounding equipment items were too severe.